Radio circuit



Aug. 30, 1332. w, L, KRAHL 1,874,309

RADIO CIRGUI-'Il Filed Deo. 6.- 1927 Vars .ymaby un@ -2 6mp HorfA/r/AL .w

xNvENToR #furie am MAH/ ATTORNEY Patented Aug.V 30, 1932 UNITED STATES PATENT OFFICE v WALTER LoUIs KRAHL, or MoNTcLaIn, NEW- JERSEY, Ass'IGNoR To ARcTURUs RADIO TUBE COMPANY, or NEWARK, NEW JEnsEmA coaronATIoN oF'nELAWAnE m10 cmcUI'r Application" mea December e, 1527. serial No. 238,923.

rlhis invention relates to a method; for controlling the hum produced in radio receiving circuits adapted too erate on tubes energized by alternating or uctuating cur'- rent.

The invention is adapted particularly to circuits employing alternating current tubes,

cations, Serial No. 140,245, filed October 8th, 1926, andSerial No. 156,946, filed December 24th, 1926. In these applications, a ther- Ref separated therefrom by vacuous space. In.A

this type of tube, it is important that the grid of each tube is connected through as low an impedance as possible to the cathode so that no great difference in alternating current potential can be built up betweenthe grid and the cathode. For radio frequency tubes, the impedence between the grid and cathode is very slight inasmuch as the grids of these tubes are connected through small inductances to the cathodes through a'D-b'attery and the impedance in the grid circuits of the audio frequency tubes is also comparatively low. However, in the detector tube, when Vgrid leak and condenser detection is employed, the grid leak provides al large impedence in the grid circuit which would normally tend to create considerable difference of potential between the grid and cathode and therefore, to produce hum.

It is one of the objects of the presentin-A vention, therefore, to provide a method for controlling the hum producedby a'detector tube using grid leak and condenser-detection. v Y p j Anotherobject of the invention is to provide a method to minimize the hum produced in such a. detector tube without substantially decreasing the sensitivity of the tube.` I i A still further object of the `invention is'to provide the means necessary to compensate for the highimpedance'in the grid circuit of a detector tube.

Other objects of the invention will be apparent as the description thereof proceeds. One embodiment of the invention has been l illustrated in Fig. 2 is a curve showing the grid; and such as illustrated in my co-pending appli- 'tentiaL Accordingly,

the accompanyingV drawing, -in

which` Fig. l'is acircuit diagram illustrating the invention;

the relation of hum in the detector tube Fig. 3 is a modification of the circuit of Fig. 1 showing another means for accomplishing the objects of the invention.

erring .more particularly to the drawing, the circuit shown in Fig.4 lfcomprises an antenna circult 10 connecting through a transformer l1 to ground at 12. The secondary 13 of the transformer 11,A following the usual practice, may have one end connected to the grid 14 of the tube I, while'thevother end is connected to a -lead 15. In like manner, the tube II may have its grid circuit 16 "connected to the lead 15, and alsoaudio frequency tubes IV and V may have their grid circuits 17 and 18 respectively, connected to the lead 15.

, As described in the above mentioned applications, one end of the heater filaments is connected tothe cathode so that this end of the filamentwill be maintained at cathode pothis end 8 ofthe filament having cathode potential on each is connected to a lead 19 which is,- A connected to the lead 15 through a D-battery 20. In this manner, all of the tubes shown in the circuit, with the exception of the detector tube IIL-has this grid circuit connected to the,... cathodev through a substantially low imped"` little if any hum is produced ance',fan'd hence, in these tubes.` Y

In the detector tube III, however, it is apparent-that by using grid leak and condenser rectification, the grid leak will intro-"i" duce alarge impedance in the grid circuit which would ordinarily produce a great potential difference between the grid and cathode, and hence, a hum. Accordingly, I have not included the grid leak in the grid circuit which passes through the transformer and 4connects to the common lead `15, as is the casewith the other tubes, but I have provided- `means to connect the circuit through the grid leak independently to the-cathode in" to the potential of tube, 1n turn,

order to compensate for the hum which would ordinarily be produced by the large impedance of the grid leak. To this end, I have provided the usual condenser 21 in the grid circuit of the detector tube III and I have connected one end of the grid leak 22 to a point between the condenser 21, and the grid of the tube and the other end of the grid leak 22, to the positive side of a battery 23 which is, in turn, directly connected to the cathode potential side 8 of the filament, asV

indicated.

The usual tuning condensers 24, 25 and 26A may be used in parallel with the secondary circuits of the transformers in the grid circuits of tubes I, II and III, respectively, and if desired, shields 27, 28 and 29 may be attached to the tubes andV grounded as indicated in the manner set forth in the above entitled applications. Lead 19 may be conlnected to one side of the secondary 30 of the power supply transformer 31, the primary 32 of which may be connected to a source 33 of alternating or fluctuating current.

The other side of the secondary 30 may be connected through a controlling resistance 34 to a lead 35, which is connected to the other end of the filament of each tube as indicated.

The other connections and devices of the circuit such as the B-battery 36, the speaker or phones 37, and the audio transformers 38 and 39, are provided in a manner well known in theV art.V A volume controllll comprising a variable resistance of from 0 to 250,000 ohms has been found desirable across the secondary of the radio frequency transformer 16, and a filament switch 4:2 may be used inthe primary 32 of the transformer 31 to turn the set on and off.

I have found by experiment that the hum produced by the detector tube has a very definite relation to the potential of the grid which relation may be easily .understood by referring to Fig. 2. In this figure, the grid potential in volts is plotted on the horizontal line, increasing negatively from Zero at the center, to 12 at the extreme left, and increasing positively toward the'right to 12vvo'lts.V

The degree of hum produced has been plotted on the verticalline, the arbitrary numerals 1, and 2, being used Afor convenience in indicating the hum factor.

I Itwill be notedV from an inspection of this curve that the greatest degree of hum is produced when the grid has a negative potential of approximately two volts, and that as the voltage of the grid approaches zero, the hum decreases very rapidly, and still continues to decrease when the voltage is increased positively. Inasmuch as the plate resistance is increased when the grid is charged negatively .so that the tube does not function properly, the right hand side of the curve must be used. However,as the positivepotential grid potential of 6 volts positive in the battery 23, using a grid leak between one and two megohms, signals are produced in which the hum is practically negligible, while the detector tube retains its sensitivity. If the resistance of the grid leak is raised or lowered, the voltage of the battery 23 may also be raised or lowered, accordingly, but I have found by experiment, that a grid leak of between one and two megohms gives satisfactory results, while a grid leak of a lower resistance, tends to lower the sensitivity of the tube..

' Another factor in decreasing the impedance between the grid and cathode, is the grid condenser 21. As the capacity of this Vcondenser is increased, the impedance in the grid circuit is decreased and hence, the hum diminished. Of course, the capacity of the condenser may not be increased to such an extent as to unbalance the circuit but I have found that capacities as large as .1 microfarad may be advantageously used with very satisfactory results.

If desired, a relatively low grid leak may be used in place of a high leak havin a positive bias, but with slightly less efficient results. I have found that grid leaks having values as low as l/n to 1/2 megohms 'may be directly connected between the grid and cathode without introducing hum, and with-` out limiting to a great extent the sensitivity of the tube.

AIn some instances, it may be desirable to use a choke in place of the grid leak and battery of Fig. 1. Such a choke is indicatedat 40 in Fig. 3 and may comprise a coil of wire wound upon an iron core. The choke offers a high impedance to radio frequencies and a low impedance to commercial lighting frequencies so that the hum factor is main-il tained at a low ligure. However, experiment has shown that the impedance should be high for audio frequencies as well as radio frequencies and as audio frequencies run as low as commercial lighting frequencies, theV results are not as desirable as with the use of a grid leak and battery.

While I have described and illustrated one embodiment of the invention, it is apparent that other circuit connections may be made without departing from the spirit of the invention, and I do not desire, therefore, to limit myself to the specific construction Having thus Vdescribed my invention, what and said grid leak in Series Vwith the grid of said tube and in parallel with the low imped# ance input circuit of the tube.

2. In a radio receiving system, a detector tube adapted to be energized by alternating current; a grid circuit including a condenser and the secondary of a transformer connected between the grid of said tube and the cathode thereof; and an auxiliary circuit, including a grid leak and a battery in series therewith connected betweenv said 'grid and cathode.

3. In a radio receiving system, a detector tube adapted to be energized by alternating current; a grid circuit includinga condenser and the secondary of a transformer connected between the grid of said tube and the cathode thereof; and an auxiliary circuit, including a grid leak of between one and two megohms and a battery of approximately six. volts in series therewith connected between said grid and cathode, said battery being connected so as to give said grid a positive potential.

4:. In a radio receiving system, a detector tube; a grid within said tube; a filament within said tube, said filament being adapted to be energized by alternating current of cominercial frequencies; a circuit connection between said grid and one side of said filament,

said connections including' an audio frequency choke; and a low impedance input circuit in parallel with said choke.

In testimony whereof, I aiix my signature.

WALTER LOUIS KRAHL. 

